Add KeyValue expression

Signed-off-by: davidarendsen <davidarendsen@hey.com>

tests/Expression/KeyValueTest.php

@@ -0,0 +1,22 @@
+<?php
+declare(strict_types=1);
+
+use Arendsen\FluxQueryBuilder\Expression\KeyValue;
+use PHPUnit\Framework\TestCase;
+
+final class KeyValueExpressionTest extends TestCase {
+
+    public function testSimpleKeyvalue()
+    {
+        $keyvalue = KeyValue::set('_measurement', 'test_measurement')
+            ->and('_field', 'user')
+            ->or('_field', 'field2')
+            ->and('user', 'my_username');
+
+        $query = 'r._measurement == "test_measurement" and r._field == "user" or ' . 
+            'r._field == "field2" and r.user == "my_username"';
+
+        $this->assertEquals($keyvalue->__toString(), $query);
+    }
+
+}